There exist several types of devices for moving tubes inside furnaces in the background art.
First, there are furnaces that allow axial movement of tubes such as, for example, barrel, gas or induction tube furnaces, with low yield and low thermal efficiency.
There exist also a great number of furnaces that use transverse movement of tubes in the background art. This movement of tubes inside the furnace is caused by rolling said tubes on a sloped plane of rails. As rolling is not ensured under all circumstances, it is necessary to prepare for manual intervention with adequate tools to ensure said rolling.
Successively, there exist other systems that to a larger or lesser extent are at present operating. For example, there is furnaces con screw tube movement, wherein displacement is ensured by screws containing the tubes supported by rails. Its practical application is limited to about 800° C. FIG. 1 shows a diagram of this type of device.
Chain furnaces are also well known, wherein tubes, supported by rails, are pushed by pushers that go through the hearth and that cooperate with the links of an endless chain, as shown in FIG. 2.
There are also “moveable bar” furnaces, wherein the tubes are supported by a series of toothed bars (fixed), while another series of toothed bars (moveable) raise, displace and then deposit the same one teeth ahead. The path of moveable bars is originally circular, as shown in FIG. 3, and has a smaller diameter than the toothed pitch of the bars, whereby the tubes rotate on each take action (on the moveable bar) and each release action, (on the fixed bar). The diameter of the path is 0.6 multiplied by the pitch, whereby a rotation of 0.2 times the pitch in the take action and of 0.2 times the pitch in the release action is attained, whereby the overall rotation is of 0.4 times the pitch per each forward movement. It is clear that the smaller is the diameter of the tube for a given pitch, the more it will rotate, and vice-versa.
Alternatively, there still are furnaces with a movement of the bars more or less rectangular with a raising mechanism and a displacement mechanism that allow, besides the forward movement as the previous system, also the movement “on site” wherein the tubes are raised to be forwarded but deposited in the same pocket, with a geometry that allows a rotation always of 0.4 times the pitch, as shown in FIG. 4.
At present, for quench furnaces, chain furnaces and moveable bar furnaces are considered suitable.
Chain furnaces went popular due to the great simplicity of its mechanism and the lesser need to provide refractory steels that are costly. However, the capability of these furnaces to quench is being analyzed due to a series of problems suffered by the pushers, originally attributed to the thermal stress suffered with the successive heating and cooling actions.
Besides this problem, it should be noted that the thermal efficiency of this type of furnaces will always be penalized by the losses incurred when heating the pushers every time.
In this context, unless very low productivities that impose very strong investment restrictions that lead to consider barrel furnaces, or chain furnaces, “moveable bar” furnaces are the natural option.
Notwithstanding moveable bar furnaces are the most adequate option for quench furnaces, there are some limitations. It is clear that efficiency of a furnace is higher when the hearth is highly occupied, that is, when the highest number of tubes is placed inside.
In a moveable bar furnace, the minimum distance between the tubes is about 15% of the tube diameter, while a reduced distance increases the residence times up to 60% being the heating period independent from the distance.
As a furnace usually has to handle different diameters, the selection of the bar tooth pitch is fundamental. It is clear that if “toothing” of the bars has a sufficiently small pitch, the loading distance of tubes can be selected to optimize the same, but it is not advisable to place the tubes on the toothing corners. In this sense, it is considered that with a toothing angle alpha, the maximum diameter D of the tube to load in a pitch “p” is D according to the following formula:D=(p−2m)/cos(alpha/2)  (Formula 1)wherein “m” is the horizontal distance from a contact point of the tube on the “V”, to the tooth corner, that is, the tolerance taken to prevent the tube from resting on the corners, as shown in FIG. 5.
Therefore when the diameter dispersion is high, being the minimum pitch determined by the higher diameter tube, it could happen that most of the tubes processed by the bar system is not optimum, that is, the furnace is not sufficiently loaded.
Thus, if one desires to improve the productivity for lower diameter tubes, the solution is to increase the length and, therefore, the cost of the furnace.
Also, moveable bars must support the total weight loaded in the furnace, so the operating machinery should have great dimensions and power, and therefore they are costly.
Another drawback/limitation of the moveable bar systems is that the tube rolling capability for shorter cycle times is very limited. The minimum displacement period of the bars, that is, to cover the complete “rectangle”, is generally from 9 to 11 seconds. This means that, for cycle times shorter than 20 to 22 seconds, the period is not enough to make “on site” movements between two forward movements.
Consequently, with low diameter and thickness tubes, the rolling movement is limited to 0.4 the pitch for each forward movement, precisely in tubes that are easily likely to twisting by anisotropy of heating.